This invention relates to a method for controlling an automatic transmission. More particularly, it relates to a method for controlling an automatic transmission wherein hydraulic servo mechanisms are controlled by means of an electronic control device, thereby controlling change of gear ratio, engagement or release of a direct coupling clutch, and the like.
Automatic transmissions loaded on automobiles are generally of the type wherein a speed change gear mechanism predominantly comprising a plurality of planetary gear mechanisms and a plurality of frictional engaging elements such as wet multi-plate clutches and brakes is coupled to the engine through a hydraulic torque converter. To compensate for a loss of power transmission in the torque converter, it is recently a common practice to arrange in parallel with the torque converter a direct coupling clutch adapted to be engaged only during driving operation above a predetermined speed. These operations of changing the gear ratio of the speed change gear mechanism and engaging or releasing the direct coupling clutch are conducted by hydraulic servo mechanisms. Two methods are known for this purpose. One method utilizes a hydraulic pressure based on the number of revolutions of an output shaft of the speed change gear mechanism and another hydraulic pressure given by a throttle valve operating in proportion to a throttle opening. The other method uses a microcomputer-based electronic control device to control solenoid valves on the basis of a throttle opening and output shaft revolutions. The latter is superior in accomplishing high precision multi-mode control.
Referring to FIG. 7, there is schematically illustrated a computer controlled automatic transmission having a direct coupling clutch. The illustrated automatic transmission is the one described in Japanese Patent Publication No. 60-32063 by the same assignee as the present invention. The automatic transmission generally designated at 1 comprises a torque converter 3 having an input shaft 2 which is coupled to the output shaft of an engine (not shown) and an output shaft 3a, a direct coupling clutch 4 disposed in parallel with the torque converter 3 for directly coupling the input shaft 2 to the torque converter output shaft 3a, and a speed change gear mechanism 7 essentially including three sets of planetary gear mechanisms and frictional engaging elements arranged between an input shaft 5 coupled to the torque converter output shaft 3a and an output shaft 6. The input shaft 5 is connected to a carrier 8 of a first planetary gear mechanism. Supported by the carrier 8 is a pinion 9 which is in mesh with a sun gear 10 and a ring gear 11. Among them, the sun gear 10 is connected so as to be integrally coupled with the carrier 8 by a F0 one-way clutch 12, and provided with a B0 brake 14 for securing the sun gear 10 to the housing. This arrangement constitutes an overdrive mechanism for setting a gear ratio of up to unity (1). The ring gear 11 on the output side is connected to an intermediate shaft 15 and a sun gear shaft 16 through a C1 clutch 17 and a C2 clutch 18, respectively. Connected to the intermediate shaft 15 is a ring gear 19 of a third planetary gear mechanism which further includes a sun gear 20 attached to the sun gear shaft 16 and a carrier 21 which supports a pinion 22 in mesh with the ring and sun gears 19 and 20. A second planetary gear mechanism includes a sun gear 23 which is also attached to the sun gear shaft 16 and a ring gear 24 which is integrally connected to the output shaft 6 along with the carrier 21 of the third planetary gear mechanism. A pinion 25 in mesh with the sun and ring gears 23 and 24 is supported by a carrier 26. The sun gear shaft 16 having the sun gear 23 mounted thereon is adapted to be selectively secured to the housing by either a B1 brake 27 or a B2 brake 29 connected via a F1 one-way clutch 28. The carrier 26 is adapted to be connected to the housing through a F2 one-way clutch 30 and to be selectively secured to the housing through a B3 brake 31.
The above-illustrated automatic transmission 1 is of the type wherein the frictional engaging elements, clutches and brakes are engaged or released by hydraulic servo mechanisms to conduct a change of gear ratios including four forward gear ratios (including the overdrive ratio) and one reverse gear ratio. An automatic transmission hydraulic control unit 32 for performing control operation for such a purpose has three solenoids Nos. 1 to 3 which are controlledly turned on and off by a control computer 33. Nos. 1 and 2 solenoids serve to control the speed change gear mechanism 7 to change the gear ratios among the four forward gear ratios and one reverse gear ratio whereas No. 3 solenoid serves to control engagement or release of the direct coupling clutch 4. Based on a number of information signals representative of vehicle driving conditions including vehicle speed, throttle opening, manual shift range, gear ratio, engine temperature, and the like, the control computer 33 computes driving parameters ensuring an optimum vehicle driving condition, and accordingly performs ON-OFF control on the respective solenoids to effect gear ratio change and engagement or release of the direct coupling clutch 4. The way of setting a certain gear ratio by the selective engagement or release of the respective frictional engaging elements is described in the above-cited patent publication. The gear ratio setting procedure is tabulated in FIG. 8. In the table, D, 3, L, R, P, and N under the heading "Manual Shift Range" represent drive range, top gear range, low gear range, reverse range, parking, and neutral; and 1st, 2nd, 3rd, O/D, and Rev under the heading "Gear Ratio" represent first speed, second speed, third speed, overdrive, and reverse gear ratios, respectively. Under the heading "Solenoids", a circle "O" means that the relevant solenoid is energized, a cross "X" means that the relevant solenoid is de-energized, and an asterisk "* " means that the relevant solenoid is energized when the direct coupling clutch is to be engaged. Under the heading "Frictional Engaging Elements", a circle "O" means that the relevant element is engaged while released elements are shown blank. Under the heading "One-Way Clutches", a triangle ".DELTA." means that the relevant clutch is engaged only during engine driving while released clutches are shown blank.
Changing to a certain gear ratio is carried out using a signal produced by the control computer 33 according to a shift schedule given by the throttle opening and the number of revolutions of the output shaft 6 as parameters. Similarly, engagement or release of the direct coupling clutch 4 is controlled using a signal produced by the control computer 33 according to a schedule given by the throttle opening and the number of revolutions of the output shaft 6 as parameters.
Now it is assumed that the accelerator pedal is worked down to increase the throttle opening. As the output shaft then gradually increases its number of revolutions, the gear ratio is progressively shifted up from the 1st ratio and the direct coupling clutch is engaged. On the contrary, the gear ratio is shifted down when the throttle opening is narrowed to reduce the output shaft revolution. If such a change of gear ratio takes place with the direct coupling clutch engaged, the changing process does not proceed smoothly, giving rise to a so-called gear change shock. A prior art common solution is to temporarily release the direct coupling clutch during gear ratio changing.
Japanese Patent Publication No. 60-32063 previously cited is directed to a control method capable of optimizing the timing of temporary release of the direct coupling clutch during the up- or down-shifting operation. Briefly stated, the method involves the steps of presetting a variety of times depending on the gear ratio changing schedules, selecting a suitable one among the preset times according to a particular gear ratio changing schedule based on changes of throttle opening and output shaft revolution, and releasing (OFF) or engaging (ON) the direct coupling clutch at a point preceding or following the time of delivery of a shift change signal by the selected time.
Change of gear ratio is carried out by changing the engagement or release of frictional engaging elements of the aforementioned speed change gear mechanism 7 by means of hydraulic servo mechanisms, and engagement or release of the direct coupling clutch carried out in a similar manner. The transient engaging or releasing properties of each element are largely affected by the temperature and viscosity of the oil used and the discharge of oil from the hydraulic servo mechanisms. For these reasons, the above-mentioned method optimizes the on-off timing of the direct coupling clutch in changing of gear ratio by presetting a variety of times.
However, in the automatic transmission 1 having the speed change gear mechanism of the type shown in FIG. 7, the state of oil in some hydraulic servo mechanisms can change with the preceding state or with time, and the stage that the relevant hydraulic servo mechanism has experienced influences the transition upon the subsequent engagement or release. FIG. 9 tabulates the operations of frictional engaging elements in the speed change gear mechanism 7 for each of the gear ratios. It is understood that clutch C2 idles in released state in the 1st range, stops in released state in the 2nd range, but is engaged in the remaining ranges. Accordingly, the volume of air taken in the hydraulic servo mechanism associated with clutch C2 (that is, the volume of oil left therein) greatly varies between these states as shown in FIG. 10. More particularly, when clutch C2 is idling in released state, the hydraulic servo mechanism associated with clutch C2 is mostly emptied of the oil. On the contrary, when clutch C2 is stopped in released state, the hydraulic servo mechanism associated therewith has a larger volume of oil left therein, with the difference in oil volume increasing with the lapse of time. Consequently, clutch C2 requires a longer time to complete its engagement when being engaged after idling than when being engaged otherwise. Differently stated, there is recognized a difference in transient properties or time required in speed change between the speed change in the order of 1st to 2nd to 3rd and the speed change in the order of 3rd to 2nd to 3rd.
Since the aforementioned prior art method, however, adjusts the timing of actuating frictional engaging elements using a predetermined fixed time, the timing adjustment does not reflect the antecedent, that is, state preceding the state from which a change is to be made. The same set time is utilized for the speed change from 2nd to 3rd irrespective of whether it is the 1st.fwdarw.2nd.fwdarw.3rd or the 3rd.fwdarw.2nd.fwdarw.3rd speed change mentioned above. Thus there is the likelihood that release or engagement of the direct coupling clutch is too fast or too late in carrying out the 2nd to 3rd speed change.
Therefore, an object of the present invention is to provide a novel and improved method for controlling an automatic transmission which can optimize not only the timing of release and engagement of a direct coupling clutch during up- or down-shifting process, but also the timing of operating frictional engaging elements without increasing the number of parts involved.